The present invention relates to an apparatus and method for driving an objective lens used to record/reproduce information with reference to an optical disk, such as a CD (Compact Disk), a CD-RW (CD-ReWritable), a DVD-ROM (Digital Versatile Disk Read Only Memory) and a DVD-RAM (DVD-Random Access Memory).
In recent years, there is an increasing demand for flat-type optical disk recording/reproducing apparatuses (optical disk apparatuses). To provide such apparatuses, very strict requirements are imposed on the design of an objective lens driving apparatus which is used for recording/reproducing information with reference to an optical disk.
For example, in the case of an objective lens driving apparatus suitable for use in a xc2xd-size optical disk having a height of 12.7 mm, the distance between the surface of the optical disk and the bottom surface of the objective lens driving apparatus must be shorter than 7.3 mm. Since the objective lens driving apparatus is installed in a restricted space, such as the space in a jacket-size apparatus or the space in a note book-included apparatus, the size of the objective lens driving apparatus is restricted in the horizontal direction as well.
In addition to the above, the objective lens driving apparatus is required to satisfy the requirements of a DVD disk, the integration density of which is seven times higher than that of an CD.
In the DVD optical system, the objective lens driving apparatus has to be very small in size and yet it must be provided with an acceleration generating function so as to ensure reliable data reproduction even when the disk is rotating fast. The objective lens driving apparatus must be also provided with such frequency characteristics as will satisfy wide-band servo specifications, because the waving and eccentricity of the disk occur at higher frequencies due to the fast rotation of the disk.
A CD system (CDxc3x971) and a DVD system (DVDxc3x971) will be compared with each other. (The xe2x80x9cxc3x971xe2x80x9d is a standard transfer speed for audio CDs.)
(1) Number of Rotations
CDxc3x971: 3.3 to 8.3 Hz
DVDxc3x971: 9.6 to 23.1 Hz
(2) Allowable Limit Values in Focusing Direction
{circle around (9)} Waving Amount (Rotation Synchronization)
CDxc3x971: xc2x10.5 mm
DVDxc3x971: xc2x10.3 mm
{circle around (2)} Waving Acceleration
CDxc3x971: 10 m/s2 
DVDxc3x971: 8 m/s2 
{circle around (3)} Allowable Residual Error
CDxc3x971: xc2x11 xcexcm
DVDxc3x971: xc2x10.23 xcexcm
(3) Allowable Limit Values in Tracking Direction
{circle around (1)} Eccentricity (Rotation Synchronization)
CDxc3x971: xc2x170 xcexcm
DVDxc3x971: xc2x135 xcexcm
{circle around (2)} Eccentricity Acceleration
CDxc3x971: 0.4 m/s2 
DVDxc3x971: 1.1 m/s2 
{circle around (3)} Allowable Residual Error
CDxc3x971: xc2x10.1 xcexcm
DVDxc3x971: xc2x10.022 xcexcm
{circle around (4)} Allowable Tilt of Objective Lens
CDxc3x971: xc2x15 mrad
DVDxc3x971: xc2x12 mrad
As can be understood from the above comparison, the characteristics regarding the waving and eccentricity of the DVD system are improved; their figures are 0.5-0.6 of those of the CD system. In spite of this, however, the allowable residual error of the DVD system is less than xc2xc of that of the CD system. This means that, due to the use of the servo system adopted, the accuracy required of the DVD system is twice as high as that of the CD system. Therefore, the high-frequency characteristic of the objective lens driving system must be expanded double, and the oscillation system including a secondary resonating unit and the electromagnetic driving system must be greatly improved, accordingly. When the rotating speed of a disk is increased, the electromagnetic driving system is required to use power with remarkably high efficiency, since the acceleration increases in proportion to the square of the number of rotations.
In the CD system, the allowable tilt of the objective lens can be realized without imposing severe restrictions on the manufacture of the CD system. In the DVD system, however, the allowable tilt of the objective lens cannot be easily realized and poses a lot of problems in the manufacture of the system, particularly when mass production of systems is considered. If the objective lens tilts, the parallelism of the objective lens to the surface of the optical disk is adversely affected. If this happens, an RF signal derived from a reflected beam from the disk decreases in level in accordance with an increase in the tilt angle, so that the reproduction or recording of data with reference to the optical disk may not be executed.
As described above, the transition to the DVD system requires an objective lens driving apparatus which is highly sensitive, covers a wide frequency range and hardly tilts, and which can be provided in a very restricted space. However, such an objective lens driving apparatus cannot be easily designed by utilizing the structure of the conventional CD system or the conventional manufacturing system. In other words, the conventional technology regarding the CD system is not very useful in producing the required objective lens driving apparatus.
Accordingly, an object of the present invention is to provide an objective lens driving apparatus which is highly sensitive, covers a wide frequency range and hardly tilts, which can be provided in a very restricted space and which has such a structure as enables further enhancement in performance. Another object of the present invention is to provide a method for manufacturing the objective lens driving apparatus.
[I] According to the first aspect of the present invention, there is provided a method for manufacturing an objective lens driving apparatus comprising: a lens holder which holds an objective lens and is driven in a focusing direction and a tracking direction with reference to the optical disk; at least two wires having first ends secured to the lens holder and controlling posture of the lens holder; and a wire holder to which second ends of the wires are fixed, the method comprising the steps of: preparing a first integrally molded member including the lens holder and the wire holder which are connected together by a holder connecting frame to be removed after assembly; preparing second integrally molded member each including the wires which are suspended from a wire holding frame to be removed after assembly; positioning the second integrally molded members relative to the first integrally molded member such that the first ends of the wires are connected to the lens holder and the second ends of the wires are connected to the wire holder; and removing the wire holding frame and the holder connecting frame.
With this structure, since the lens holder and the wire holder are prepared as one integrally molded member, the wires can be attached to them in such a manner that the positional relationships which they have immediately after they are molded are maintained. Moreover, since the wires suspended by the wire holding frame are prepared as integrally molded members, the wires, even though they are thin, can be easily assembled without damage.
According to one embodiment, the wires, the lens holder and the wire holder are positioned relative to one another by bringing the holder connecting frame into contact with the wire holding frame.
[II] According to the second aspect of the present invention, there is provided an objective lens driving apparatus incorporated in an optical disk apparatus which is for recording or reproducing information from an optical disk, the objective lens driving apparatus comprising: a lens holder which holds an objective lens and is driven in a focusing direction and a tracking direction with reference to the optical disk; at least two wires having first ends secured to the lens holder and controlling posture of the lens holder; and a wire holder to which second ends of the wires are fixed, the wires being suspended by a wire holding frame which is to be removed after assembly, and the wires being first secured to the lens holder and the wire holder and then the wire holding frame being removed.
With this structure, since the wires suspended by the wire holding frame are prepared as integrally molded members, the wires, even though they are thin, can be easily assembled without damage.
According to one embodiment, the lens holder and the wire holder are held in such a manner that they are connected by the holder connecting frame which is to removed after assembly. After the wires are secured to the two holders, the wire holding frame is removed, thus allowing the two holders to move relative to each other. The wires are positioned with reference to the lens holder and the wire holder by positioning the wire holding frame with reference to the holder connecting frame.
With this structure, since the lens holder and the wire holder are prepared as one integrally molded member, the wires can be attached to them in such a manner that the positional relationships which they have immediately after they are molded are maintained.
According to one embodiment, the lens holder is provided with first bosses to which the first ends of the wires are to be fixed. The first ends of the wires have first insertion holes. After being loosely inserted into the first insertion holes, the first bosses are fixed therein by means of an adhesive. The wire holder is provided with second bosses to which the second ends of the wires are to be fixed. The second ends of the wires have second insertion holes. After being loosely inserted into the second insertion holes, the second bosses are fixed therein by means of an adhesive.
With this structure, the wires can be reliably bonded to the lens holder or the wire holder, with no need for direct adhesion between them.
According to one embodiment, the wire holder comprises a main body, and swinging sections integral with the main body. The second ends of the wires are secured to the swinging sections. One end of each swinging section is fixed to the main body by means of thin members, while the other end thereof extends to the region in the vicinity of the main body. It is preferable that a vibration suppressing member be interposed between the main body and the other end of each swinging section.
With this structure, since the swinging sections used for holding the second ends of the wires are integral with the wire holder, the wires can be assembled with high accuracy, which corresponds to the accuracy at which they are molded. Since this structure is effective in suppressing the resonance of the wires, excellent servo characteristics are ensured at high-frequency regions.
It is preferable that the above-mentioned swinging section be provided for each of the wires. However, if it is provided for one of the wires, resonance can be significantly suppressed.
According to one embodiment, the wire holder is provided with a gel reservoir located at a position corresponding to an intermediate portion of the wire. A gel member is sealed in the gel reservoir and serves to prevent the wire from vibrating.
The structure described above is effective in preventing the wires from buckling. In many cases, the vibration due to high-order resonant frequencies is transmitted in the direction in which the wires extend, and this vibration is likely to cause buckling of wires. According to the present invention, the gel described above serves to suppress the vibration at the middle point of each the wire, and the vibration can therefore be suppressed.
According to one embodiment, the lens holder is provided with a gel reservoir located at a position corresponding to an intermediate portion of the wire. A gel member for preventing vibration of the lens holder is sealed in the gel reservoir.
The structure described above is effective in preventing the lens holder from vibrating at high-order resonant frequencies.
According to one embodiment, the objective lens driving apparatus further comprises: a focusing coil, provided for the lens holder, for driving the lens holder in the focusing direction; a pair of tracking coils provided for those portions of the lens holder which are ends, as viewed in the tracking direction; and a pair of printed wiring boards, provided for those portions of the lens holder which are ends, as viewed in the tracking direction, and having wiring connected to the focusing coil and/or the tracking coils, the printed wiring boards being connected together by means of a board holding member which is to be removed after assembly, and the printed wiring boards being first secured to the lens holder and then the board holding frame being removed.
With this structure, since the printed wiring boards need not be positioned relative to each other, they can be easily assembled. In addition, since the board holding member is-removed after assembly of the printed wiring boards, movement of the lens driving apparatus need not be affected thereby.
According to one embodiment, the objective lens driving apparatus further comprises: a focusing coil, provided for the lens holder, for driving the lens holder in the focusing direction; a pair of tracking coils provided for those portions of the lens holder which are ends, as viewed in the tracking direction; and a flexible printed wiring board, provided for the wire holder and having external-connection wiring connected to the focusing coil and/or the tracking coils, the flexible printed wiring board having an intermediate portion which is intermediate as viewed in the tracking direction and which is reduced in width.
Since the intermediate portion of the flexible printed wiring board has a reduced width, the wire holder (which serves as a swinging section) is not prevented from moving.
According to one embodiment, the objective lens driving apparatus further comprises: a focusing coil, provided for the lens holder, for driving the lens holder in the focusing direction; a pair of tracking coils provided for those portions of the lens holder which are ends, as viewed in the tracking direction; and a flexible printed wiring board, provided for the wire holder and having external-connection wiring connected to the focusing coil and/or the tracking coils, the flexible printed wiring board having a slit formed in a portion which is intermediate as viewed in the focusing direction.
Since the intermediate portion of the flexible printed wiring board has a slit, the wire holder (which serves as a swinging section) is not prevented from moving.
According to one embodiment, the objective lens driving apparatus further comprises: a chassis to which the wire holder is fixed; an electromagnetic driving mechanism, arranged between the chassis and the lens holder, for driving the lens holder; and an actuator cover formed of a material having high magnetic permeability, coupled to the chassis and covering movable portions of the wire and the lens holder.
It is preferable that the chassis and the actuator cover be held at least four positions. For example, the cover and the yoke provided for the chassis are preferably kept in contact with each other in such a manner that the yoke holds the actuator cover.
Since this structure enables the cover to be adsorbed by the chassis, the vibration of the cover is prevented. Since the vibration of the cover is prevented, the fluxes between the magnetic poles are prevented from varying.
According to one embodiment, the objective lens driving apparatus further comprises: a chassis to which the wire holder is fixed; and an electromagnetic driving mechanism, arranged between the chassis and the lens holder, for driving the lens holder, the chassis having a depression formed in a surface opposite a surface for which the wire holder is provided,
Where this structure is used for adjusting the tilt angle in the tangential direction of the lens, the wire holder is provided with a stepped portion at a position away from the objective lens. Such a stepped portion enables the adjustment to be made with a high degree of freedom.
The structure described above is also advantageous in that the lens holder is allowed to move in a wide range with no need to employ a thick objective lens driving apparatus.
According to one embodiment, the lens holder and the wire holder are preferably suspended by reduced-width, thin suspension members. The suspension members are preferably designed such that they can be easily cut off or removed. For example, they are half cut. To provide half-cut structures, pressing or etching may be utilized. If the suspension members are made of plates having a thickness of 80 xcexcm, the half-cut portions may be 30 xcexcm or so. When pressing is utilized, the half-cut portions are provided by forming V-shaped grooves. When etching is utilized, they are provided by forming a U-shaped groove.
It is preferable that the suspension members extend in the same direction as the wires. Where this structure is employed, the wire holding frame can be removed without subjecting the wires to an external force.
The suspension members are preferably located at positions corresponding to weld lines with reference to the position where the mold resin is injected from the mold. When this structure is adopted, the resin flows toward the suspension members after reaching the weld lines. Since the weld lines are deformed and set in the fitted state, the mechanical strength of the weld-line portions is improved, and that of the entire holder is also improved.
[III] According to the third aspect of the present invention, there is provided an objective lens driving apparatus incorporated in an optical disk apparatus which is for recording or reproducing information from an optical disk, the objective lens driving apparatus comprising: a lens holder which holds an objective lens and is driven in a focusing direction and a tracking direction with reference to the optical disk; at least two wires having first ends secured to the lens holder and controlling posture of the lens holder; and a wire holder to which second ends of the wires are fixed, the wire holder having swinging sections which are integral therewith and to which the second ends of the wires are fixed.
According to this structure, the swinging sections, which hold the second ends of the wires, are integral with the wire holder. Accordingly, the wires can be assembled with such high accuracy as corresponds to the accuracy at which they are molded. This structure is effective in suppressing the resonance of the wires, so that excellent servo characteristics are ensured at high-frequency regions.
[IV] According to the fourth aspect of the present invention, there is provided an objective lens driving apparatus incorporated in an optical disk apparatus which is for recording or reproducing information from an optical disk, the objective lens driving apparatus comprising: a lens holder which holds an objective lens and is driven in a focusing direction and a tracking direction with reference to the optical disk; at least two wires having first ends secured to the lens holder and controlling posture of the lens holder; and a wire holder to which second ends of the wires are fixed, the lens holder having first bosses to which the first ends of the wires are to be fixed, the first ends of the wires having first insertion holes into which the first bosses are loosely inserted, the first bosses and the first insertion holes being fixed together by means of an adhesive, the wire holder having second bosses to which the second ends of the wires are fixed, the second ends of the wires having second insertion holes into which the second bosses are loosely inserted, and the second insertion holes and the second bosses being fixed together by means of an adhesive.
With this structure, the wires can be reliably bonded to the lens holder or the wire holder, with no need for direct adhesion between them.
[V] According to the fifth aspect of the present invention, there is provided an optical head apparatus comprising the objective lens driving apparatus according to one of the second to fourth aspects. There is also provided an optical disk apparatus comprising the optical head apparatus.
The structure of the fifth aspect is effective in suppressing the tilt phenomenon, and the optical disk apparatus comprising this optical head apparatus suppresses read and write errors to the maximal degree.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.